Light Dependent Resistors (LDRs)

Objectives:

• Understand how a light-dependent resistor (LDR) works. • Be able to describe the relationship between resistance and light intensity for an LDR. • Be able to describe applications of LDRs.

Task 1 - Starter

Remind the person next to you:

• The relationship between voltage and current for a resistor.

• The relationship between temperature and resistance for a bulb.

• The relationship between temperature and resistance for a thermistor.

Task 2

You are going to investigate how the resistance of an LDR changes with light intensity.  Your teacher will give you a multimeter and an LDR.  You will need to collect a ray box, power supply, a multimeter and some leads.  Test the resistance of the LDR as you increase the voltage of the lamp  from 0V to 12V.  You will need to draw a suitable table to collect your results.

Voltage across lamp (V) Resistance of LDR (kΩ) 0   1   2   3   4   5   6   7   8   9   10   11   12

Task 3

Plot a graph of your results.

• Lamp voltage should be along the x-axis.

• Resistance should be along the y-axis.

• Both the axes should have labels and units.

• Give your graph a suitable title.

Your graph should look something like this!

Task 4

Listen carefully as your teacher shows you this presentation, and explains why an LDR behaves like this.

Homework/Extension:

Find out at least 3 places that thermistors are used to control or measure temperature.  Write a sentence to explain how the thermistor is used for each application.

Find out at least 3 places that LDRs are used to control or measure light intensity.  Write a sentence to explain how the LDR is used for each application.